But federal tax tables that guide those withholdings did not account for some wage earners whose personal situations complicated the tax credit calculation. That category included some workers with more than one job, some married couples in which both spouses work, and some Social Security recipients with jobs.

The Treasury Department’s inspector general for tax administration estimated that more than 10 percent of all taxpayers who file individual tax returns for 2009 could owe additional taxes because of the complexities of the Making Work Pay credit. Of that group, the IRS estimated that 65,000 taxpayers could technically face penalties for underpaying their taxes in 2009, although the agency said it would waive such fees.

While the inspector general’s report estimated that millions of taxpayers will be negatively affected by the tax credit, a Treasury official said the vast majority will only receive a slightly reduced refund and not an out-of-pocket tax liability on April 15.

Of course, at 1419 pages long, who had the time to read all the fine print in the stimulus bill, let alone peruse IRS withholding tables?